In the process of mounting and sealing air-blown microtubules or fiber tubes within a floor-type optical cable connection box, it is necessary to first pass the casings upwards one by one through a rubber ring. However, since the sponge rubber ring and the fiber tubes are usually soft and the diameter of an opening formed in the sponge rubber ring is often smaller than the diameter of the fiber casing, it is difficult to insert the fiber casing into the opening within the sponge rubber ring. Furthermore, if a plurality of fiber tubes are inserted into or fixed to the sponge rubber ring, a larger resistance has to be overcome when the relative position between the sponge rubber ring and the fiber casing is later adjusted, which makes the adjustment operation difficult.